It is known that, in general, in the electrolysis of aqueous solutions of chlorides, at the anode chlorine is developed, and the cathodic reaction can either be the development of hydrogen with production of alkalinity, or the precipitation of the metal, according to the position the latter occupies in the series of the electrochemical potentials, according to the following reactions:
anodic reaction: EQU Cl-e-&gt;1/2Cl.sub.2
cathodic reaction: EQU Me.sup.+ +e+H.sub.2 O--&gt;MeOH+1/2H.sub.2
or EQU Me.sup.+ +e--&gt;Me
At acidic pH values, chlorine gas is developed.
Under neutral or alkaline pH conditions, chlorine, owing to the increase in its water solubility, causes, by dismutation, the formation of hypochlorite and other oxygen-containing compounds, such as chlorate and perchlorate.
In the case of alkali-metal chlorides at pH&lt;4, chlorine is produced, and at higher pH value alkali-metal hypochlorites or, in the case of higher anodic potentials, alkali-metal chlorates and perchlorates are produced.
Large amounts of chemical products are manufactured by this route.
In the case of heavy metal chlorides (Cu, Co, Ni, Zn, Cd, Pb, etc.), at a relatively acidic pH, the metal is deposited at the cathode and chlorine is developed at the anode.
The anodic compartment of the cell must be kept separated from the cathodic compartment by means of a diaphragm or a membrane, and the anodic compartment should be closed in order to make it possible for pure chlorine to be collected, first of all in order to prevent so toxic a gas from getting dispersed in the environment, and, furthermore, in order to prevent chlorine from coming, by diffusion, into contact with the deposited metal, and dissolving it.
The split cell, the use of which is mandatory for this kind of process, adds a considerable complication to the electrolysis facility and, in the event when an ionic membrane is used in order to separate the compartments, it also implies a very high equipment cost.
The production of chlorine, parallel to metal production, constitutes another limitation to the application of the electrolysis of chlorides for producing metals, because it is necessary that the same process can make Use of the chlorine it produces.
This is the case, for example, in the Falconbridge process, which produces electrolytic nickel from aqueous solutions of chlorides and uses chlorine in order to oxidize the ore.
In general, according to the prior art, the electrolysis of the aqueous solutions of heavy metal chlorides did not enjoy those important industrial applications which its potentialities would deserve given the advantages it offers on energy side, due to the high conductivity of chloride solutions, and given the anodic potential of chlorine development being lower than of oxygen development.
The alternative solutions to the anodic chlorine development adopted heretofore are, e.g., the oxdiation of Fe.sup.2+ to Fe.sup.3+, or of Cu.sup.+ to Cu.sup.2+ which, by occurring at a lower potential than of chlorine development reaction, avoid the production of the latter, and offer an advantage as regards the cell voltage. An example is the clear process, according to which in the cathodic compartment Cu is deposited, and at the anode iron and copper are oxidized: these, in their turn, are used in order to oxidize chalcopyrite, converting sulphide into elemental sulphur and dissolving copper.
Another solution adopted is of using in the anodic compartment a solution of an oxyacid, e.g., sulphuric acid. In this case, in order to separate the anodic from cathodic compartment, an ionic membrane, and the anodic reaction turns into a water oxidation one: EQU H.sub.2 O-2e.fwdarw.1/2O.sub.2 +2H.sup.+.
At the anode, oxygen is developed, and H.sup.+ ions, through the membrane, reach the cathodic compartment.
Summarizing the present state of the art of metal electro winning from chloride solutions, one may state that, in the case of chlorine production, as well as in the case of alternative anodic reaction, a cell split by a diaphragm an ionic membrane should be always used, with all of the facility complications and the higher costs involved by such a structure.